Telephone substation circuits



July 3, 1928.

R. s. OHL

TELEPHONE SUBSTATION cmcurrs Filed June 24, 1926 INVENTOR 1?. 5. 0106 ATTORNEY Patented July 3, 1928.

UNITED STATES PATENT OFFICE.

RUSSELL S. OHL, OF NEW YORK, N. Y., ASSIGNOR T0 AMERICAN TELEPHONE AND TELEGRAPH COMPANY, .A. CORPORATION OF NEW YORK.

'rnnnrnonn SUBSTATION oIRcUI'rs.

Application filed June 24,

This invention relates to telephone substation circuits, and particularly to arrangements for eliminating the detrimental effects ontelephone substation circuits caused by high frequency currents.

A subscribers substation circuit may occasionally' be so located that it is exposed to a strong high frequency electromagnetic field. Dueto this exposure, high frequency currents may flow through various portions of the substation circuit. Such currents may result either from induced high frequency electromotive forces established around any mesh of the electrical network, or from capacity imbalances to ground in various portions of such electrical network. The flow of high frequency current through the microphone of the substation circuit may lead to interference between the high frequency field and the voice frequencies which normally operate the substation circuit. This interference is caused by the microphone acting as a detector by virtue of the rectifying properties of its carbon button. When these high frequency currents become modulated by speech originating at a radio transmitting station, the speech will often be heard at the'telephone receiver of the substation circuit, the speech very often being sufficient in volume to impair the normal communicative qualities ofthe telephone system. However, if the high frequency radio Waves are initially unmodulated by speech, these high frequencies may nevertheless become modulated by speech transmitted over the telephone lines, when such speech inipinges upon the diaphragm of the transmitter of the substation circuit. ,In thiscase the telephone conversation can be picked up y radio receive'rswhichare tuned to receive the radiofrequencywaves, these radio frequency waves being re-radiated, modulated by the telephone conversation. Consequently those operating the radio receivers hear the telephone conversation. I t

This same effect has been experienced where the subscribers loop was employed as the radio antenna system. When the telephone set was connected to a radio receiver, perhaps through a condenser, the subscribers loop and the telephone set thus acting asa substitute for the usual receiving antenna system, high frequency currents set up in the subscribers circuit passed through the microphone. W hen the receiver was re- 1926. Serial No. 118,349.

moved from the switch hook to permit a telephone conversation to take place, speech passing through the telephone set was superimposed on the high frequency ener y, whereupon it was demodulated at the rat io receivers tuned to the high frequency energy, so that theflaudience at the radio receivers was again able to hear the telephone conversation.

Accordingly,'it is one of the objects of this invention to eliminate the detrimental effects produced by electromagnetic waves superimposed on a telephone substation circuit.

Another of the objects of this invention is to alter the well-known substation circuit so as to render it substantially free from the detrimental effects of high frequency currents, without causing any substantial change in the voice frequency transmission characteristics of the substation circuit.

And it is a further object of this invention to prevent the flow of high frequency currents in a substation circuit, or in a portion thereof, which circuit may be connected to a telephone line exposed to a high frequency electromagnetic field.

\Vhile theinvcntion will be pointed out with particularity in the appended claims, the invention itself,both as to its further objects and features, will be better under stood from the detailed description hereinafterfollowing when read in connection with the accompanying drawing, in which Figure 1 shows one form of substation circuit, Fig. 2 shows an arrangement for substantially preventing the effects mentioned hcrcinabove, and Figs. 3 and 4 show pal ticular ways in which the substation circuit may be modified so as to eliminate these effects.

Referring to Fig. 1 of the drawing, conductors A and 13 represent conductors connected to a tclephoneline which may be assumed to be connected to another substation circuit (not shown) through one or more central offices. A. ringer R and a condenser. C are in series relationship with each other at the substation. It will be understood, however, that the proper ringing current for operating the ringer R- may be impressed conductor B. A receiver P is associated with the substation circuit so that speech transmitted over the telephone conductors A and B originating at a distant substation circuit may be received by the receiver P. Speech may be transmitted over the conductors A and B to a distant substation circuit by means of the transmitter T. The local transmitting circuit includes the conductor A, the winding the transmitter T and the conductor 13. The local receiving circuit includes the winding IV, and the receiver P. Signals coming in over the conductors A and B are first impressed 011 the winding W By virtue of the inductive relation existing between windings \V and WV these signals are heard at the receiver 1.

When the conductors of the telephone line are exposed to a high frequency electromag netic field, high frequency currents may flow in the telephone line, these currents passing over the conductors A and B. These high frequency currents then pass through the winding IV, and the transmitter T. As the 1 high frequency energy passes through the transmitter T, it becomes rectified by virtue of the rectifying properties of the carbon button of the transmitter T. The rectified current is impressed upon the winding N whereupon it is received at the receiver P because of the mutually inductive effect between windings IV and VV,.

This invention is directed to provide arrangements for substantially suppressing high frequency energy so that this high frequency energy is not detected at the substation circuit, and so that the effect thereof is substantially negligible at the receiver 1? of the substation circuit. This high frequency energy may be suppressed in a number of ways, one of which involves the introduction of suitable induc-tances in the leads connected to the transmitter T, these inductances being of magnitudes sutlicient to substantially supress the high frequency current which would otherwise flow through the transmitter T. The transmitter itself is a low c-mpedance device and offers practically no resistance to the flow of this high frequency current. These inductances, howev r, greatly increase the impedance in series with the transmitter T so that no appreciable high frequency current can pass therethrough. Moreover, these inductances are, preferably, of low distributed capacity, for a high distributed capacity would normally decrease the impedance in series with the 'arbon button of the transmitter T.

In Fig. 2 there are shown two windings IV and IV each of suitable inductance, connected between the windings IV, and VV, respectively, and the terminals of the trans niitter T. The wine 11;: IV, is in series with the winding IV ant the winding W is in series with the winding Moreover, these inductances may be located a considerable distance from the box, or other container, in which the windings IV, and IV, are housed.

Fig. 3 shows the windings V and IV, connected in a manner similar to that shown in Fig. 2; yet in this arrangement, the wind ings W, and IV, are intended to be included along with windings IV, and IV in the normal substation circuit. The box which houses windings IV, and IV, also houses windings IV, and IV In Fig. l there is a slight modification of the arrangement shown in Fig. 3. In this figure, the ringing circuit includes the conductor A, the winding IV,,, the ringer R, the condenser C, windings \V and W. and conductor B. The transmitting circuit includes the. conductor A, the winding W the winding V the transmitter T, the winding IV, and the conductor B. The receiving circuit includes the windings V and V and the receiver P.

Vhile the invention has been shown in certain particular embodiments for the pur-v pose of illustration, it is to be understood that the general principles of this invention may be applied to other and widely varied organizations without departing from the spirit of the invention or the scope of the appended claims.

IVhat is claimed is:

1. A system for eliminating the effect of high frequency currents induced in a telephone substation circuit, comprising means within said telephone substation circuit and in series with its transmitter for substantially suppressing these high frequency currents by reactances of predetermined magni: tude.

2. In a telephone substation circuit comprising a transmitter, a receiver and a ringer, the combination of reactance means in series with the transmitter so as to prevent its action as a rectifier of high frequency currents flowing in the substation circuit.

3. In a telephone substation circuit comprising a transmitter, a receiver and a ringer, the combination of a telephone line, and means including choke coils in series with said transmitter to prevent its action as a rectifier of high frequency currents flowing in said telephone line. v

4;. In a telephone substation circuit. comprising a transmitter, a receiver and a ringer, the combination of means in series with said transmitter interposing high impedance to radio frequencies for substantially suppressing induced modulated and unmodnlated radio frequency energy from said substation circuit.

5. In a telephone substation circuit comprising a transmitter, a receiver and a ringer, the combination of a telephone line, and inductance means in series with said trans mitter for substantially preventing the reception of a conversation taking place over said telephone line through any high frequency transmission system exposed to said telephone line.

6. In a telehone substation circuit, the combination 0 a telephone line, two mutually reactive inductances, a condenser and a ringer in series relationship with said telephone line, a transmitter connected to said telephone line through one of said inductances, a receiver connected to said telephone line through the other of said inductances, and choking means also serially connected to said transmitter for substantially preventing the modulation of high frequency currents by the normal currents transmitted over said telephone line.

7. In a telephone substation circuit, the combinatin of a telephone line, two mutually reactive inductances, a condenser and a ringer in series relationship with said telephone line, a transmitter connected to said telephone line through one of said inductances, a receiver connected to said telephone line through the other of said inductances, and additional reactance means in series with said transmitter to prevent its action as a rectifier of radio frequency currents superimposed on said telephone line.

8. In a telephone substation circuit, thecombination of a telephone line, two mutually reactive inductances, a condenser and a ringer in series relationship with said telephone line, a transmitter connected to said telephone line through one of said inductances, a receiver connected to said telephone line through the other of said inductances,

and radio frequency chokes also in serles with said transmitter for substantially preventing induced radio frequency energy from flowing therethrough.

.9. In a telephone substation circuit, the combination of a telephone line, two mutuallyreactive inductances, a condenser and a ringer in series relationship with said telephone line, a transmitter connected to said telephone line through one of said inductances, a receiver connected to said telephone line through the other of said inductances, and choking means also in series with said transmitter and having a definite inductive value for substantially preventing the transmission of a conversation over said telephone line through any high frequency transmit,

ting array exposed to said telephone line.

In testimony whereof, I have signed my name to this specification this 23rd day of June, 1926.

RUSSELL S. OHL. 

